Unless explicitly indicated herein, the materials described in this section are not admitted to be prior art.
There are many different position detection systems used in many fields of technology to detect the position of a target object in a particular frame of reference. One particular class is magnetic position detection systems in type of which a target object is either magnetised or carries magnets whose field is detected by one or more magnetometric sensors. The detected field varies with the position of the target object relative to the sensors and so the position of the target object can be detected. An example of an industrial use of such a system is found in U.S. Pat. No. 6,690,159 in which a target device incorporates a magnet, and a linear sensing device incorporating an array of Hall effect sensors is provided which have outputs that vary as a function of the strength of the magnetic field induced by the magnet in the target device. This can be used, for example, in a vehicular wheel speed sensing system, in detecting the position of internal components of a machine or in the detecting of the attitude of one part of a machine relative to another. Magnetic position detection has also been proposed for use in the medical field. For example U.S. Pat. No. 8,216,149 and WO-A-2012/040077 disclose the use of magnetic position detection to detect the movement of a needle within a needle guide attached to an ultrasound imaging probe.
In all magnetic positioning detection systems the accuracy of the system is dependent upon the output of the magnetometric sensors varying only with the change in position of the tracked object and not being corrupted by the effect of varying stray magnetic fields from other sources. In some systems the effect of stray fields can be avoided by ensuring that stray fields are much smaller than the field from the target object, for example by shielding the detection space of the position detection system and/or by ensuring that the magnetic field from the target object is relatively high, for example by using a high magnetisation or having the target object and magnetometric sensors in close proximity. Also if the magnetometric detectors are in a fixed position and the magnetic field is not time varying, the effect of stray fields can be eliminated by simply measuring the magnetic fields in the absence of the target object, and subtracting these values from the subsequent measurements when the target object is present.
More recently, however, magnetic position detection has been proposed for use in more challenging applications. For example, in the field of ultrasound image guided surgical procedures in which a tissue penetrating medical tool or instrument such as a hypodermic needle, cannula, catheter or the like is inserted into the patient while ultrasound imaging a target anatomical region within the patient, it has been proposed to enhance the tracking of the medical tool by magnetising it and detecting its position relative to the ultrasound probe by means of a magnetometric detector attached to the ultrasound probe. Such a system is described in our co-pending International (PCT) patent application PCT/EP2011/065420. In this system the tissue penetrating medical tool, e.g. the needle, is magnetised and an array of magnetometers on the freehand ultrasound probe detect the magnetic field from the needle and, of course, a background which is mainly the terrestrial magnetic field. The position of the needle relative to the magnetometric detectors, and thus the ultrasound probe, is calculated by modelling the combination of the terrestrial magnetic field and the needle's magnetic field and fitting the model to the magnetometric sensor measurements. This gives the position of the needle relative to the probe and this magnetically detected position can then be displayed superimposed on the ultrasound image display of the patient's anatomy. Advantages of this system include the fact that the ultrasound probe is a freehand probe which can thus be freely manipulated by the operator to image the target anatomy in the best way, and that the target object, for example the needle, is a standard one which has been magnetised. This means that the familiarity and experience with which the user has built up with handling such standard tools is preserved. Such experience and familiarity is important with invasive procedures, such as regional anaesthesia, vascular access, fine needle aspiration, musculoskeletal injections and so on in which the operator often relies on tactile feedback (“feel”) to guide the procedure, for instance to know when the desired target anatomy is reached by the needle.
However these features lead to disadvantages in trying to cope with the effects of stray magnetic fields. The fact that the magnetometric detectors are not fixed in position means that spatial variation in the background magnetic field will affect the measurements. Further, the magnetic field from the target object may be weak, meaning that stray magnetic fields tend to have a greater influence.
Although it would be possible to eliminate the effect of background magnetic fields by measuring them and subtracting them, simple and practical ways of doing this are not easy to find.
It would be therefore be desirable to provide a magnetic position detection system with a way of automatically detecting whether or not the target object is present, which would allow accurate zeroing. It is also desirable to provide for automatic zeroing on the basis of such a determination and preferably to allow an indication to be given to the user the degree of confidence in the position detection.
The present invention provides a magnetic position detection system comprising: a magnetic target object, an array of magnetometric sensors for detecting the magnetic field from the target object in a detection space, and a data processor for receiving magnetic field measurements from the array of magnetometric sensors and calculating the position of the target object relative to the array, the data processor being adapted to detect whether or not the target object is present in the detection space by: fitting a magnetic field model to the magnetic field measurements by minimising differences between the magnetic field model and the magnetic field measurements, determining remaining differences between the fitted magnetic field model and the magnetic field measurements, and determining from the remaining differences whether or not the target object is present in the detection space.
The sensors may be calibrated to provide calibrated values as said measurements. Optionally the average magnetic field measured by the sensors, or a subset of them, may be subtracted from the measurements, and the model fitted to the resulting deviations from the average. The terminology “magnetic field measurement” used herein may be therefore the calibrated measurements from calibrated sensors or deviations of them from the average field obtained by calculation.
Preferably the target object is rendered magnetic by carrying one or more permanent magnets or by at least part of it being permanently magnetised itself. Alternatively the target object could be magnetised from outside by induction.
The invention is particularly applicable to a position detection system in which the target object is a tissue-penetrating medical or surgical tool or instrument such as a needle, cannula, catheter or stylet. The system may be used in an ultrasound image guided procedure by having the magnetometric sensors attached to a freehand ultrasound probe of an ultrasound imaging system. Preferably the magnetically detected position of the target object is then displayed on the image display of the ultrasound image system. Preferably the position of the target object relative to the array of magnetometric sensors is determined by modelling the magnetic field from the target object, optionally with a background magnetic field comprising the terrestrial magnetic field. The modelled background magnetic field may comprise magnetic field influences from other magnetic objects.
The magnetic field from the target object may be modelled as plural spaced monopoles and a model is preferably a parametric model including as a parameter the position of the target object relative to the magnetometric sensors.
Preferably the step of determining whether or not the target object is present in the detection space comprises comparing the remaining differences after model fitting to the magnetic field measurements themselves. For example the sum of the absolute differences or squared differences, or the ratio of these quantities to the mean of the magnetic field measurements (or their squares), optionally normalised to the number of field measurements, may be calculated and compared to a threshold. If the differences after model fitting are high compared to the magnetic field measurements then it suggests the measurements are not accurately fitting the model, which in turn suggests that the target object (whose field the model represents) is not present. On the other hand if the differences are low compared to the measurements, it suggests that the model is accurately fitting the measurements suggesting that the target object is present. Thus the comparison of the differences between the fitted magnetic field model and the magnetic field measurements provides a metric which indicates whether or not the target object is present.
Preferably detection of a target object is confirmed (or not) by also examining whether other parameter values returned by the model are realistic, e.g. whether the length of the target object is within a tolerance, e.g. 50%, or more preferably 20-25%, of the actual target object length, whether the distance between the target object and the sensor array is more than a preset amount—e.g. 120 mm, whether the magnetisation is within a preset tolerance, e.g. a factor of two, of what is expected. Also whether the residual is below a preset threshold and whether the model fitting process takes less than a preset time can be used to conform whether or not a target object is present. These are supervisory checks which can be used individually or in combination.
This metric for determining whether or not the target object is present can be utilised in a position detection system to provide for reliable automatic zeroing. This is because an accurate and robust determination that the target object is not present allows the magnetic field measurements from the magnetometric sensors to be taken as accurately representing the background field (including any stray magnetic field, in the detection space). Such background measurements can be stored and then used in the position sensing process for the target object. In the position sensing process the magnetic field measurements from the magnetometric sensors can be corrected by removing from them the influence of the stored background measurements and then a model of the magnetic field from the target object can be fitted to the corrected measurements.
Preferably the target object position sensing process is conducted more frequently than the background field measurement process. Thus, for example, several successive cycles of target object position sensing can use the same stored background measurements. The background measurements will preferably only be updated upon determination that the target object is no longer in the detection space.
Another aspect of the invention provides an indication to the user on whether stray magnetic fields are present in the detection space by calculating the spread of the background magnetic field. This gives an indication of the confidence in the accuracy of the magnetic position detection. If the spread is low, the stray fields are not present or small enough to be ignored. If larger, then the user can be advised by an indicator to hold the probe steady. If still larger then the user can be advised that the magnetic position detection is not confident. The spread can be the standard deviation or some other measure of spread such as the average over the magnetic field sensors of the deviation of their values from the mean background field
Thus in more detail this aspect of the invention provides a magnetic position detection system comprising: a magnetic target object, an array of magnetometric sensors for detecting the magnetic field from the target object in a detection space, and a data processor for receiving magnetic field measurements from the array of magnetometric sensors and calculating the position of the target object relative to the array by fitting a model of the magnetic field from the target object to the magnetic field measurements, the data processor being adapted to calculate an indicator of the confidence in the calculated position by: measuring the magnetic field in the detection space in the absence of the target object to obtain a set of magnetic field measurements; calculating the spread of the magnetic field measurements, and displaying to a user an indicator based on the spread to indicate the confidence in the calculated position.
The spread may be the standard deviation of the amplitude of the magnetic field measurements, or another statistical measure of spread, e.g. the average deviation from the mean, and the data processor may be adapted to compare the spread of the measurements to two predetermined thresholds to indicate three conditions, or the indicator may give a more continuous indication of spread and thus confidence.
The data processor may adapted to detect whether or not the target object is present in the detection space as explained above. Thus the different aspects of the invention are advantageously combined together in a single system, though each of them may be used separately. For example the background field can be measured in a different way and used to generate a stray field indicator, or the absence of the target object can be determined in a different way in the generation of the stray field indicator.
The invention also extends to an ultrasound imaging system comprising a magnetic position detection system as explained above.
The invention also provides a corresponding method of determining whether or not a target object is present in the detection space of a magnetic detection system, a corresponding zeroing method to eliminate the effects of stray magnetic fields, and a corresponding method of indicating to a user the confidence in the magnetically-detected position.